UC DAVIS (US) — By comparing meteorites to Earth rocks, researchers have solved the mystery of the planet’s missing chromium—and gained a clearer picture of early Earth.

It has been known for decades that chromium is relatively underrepresented in the Earth’s mantle and crust, says Qing-Zhu Yin, professor of geology at the University of California, Davis. That could either be because it evaporated into space, or got sucked into the Earth’s deep core at some point.

Using specialized equipment, Yin and colleagues made very exact measurements of chromium isotopes in meteorites and compared these to rocks from the Earth’s crust. With modern, high-performance computers, they then used the data to generate a simulation of Earth’s early environment.

The team studied a class of meteorites called chondrites, which are leftovers from the formation of the solar system more than 4.5 billion years ago.

As well as adding shiny, rust-proof surfaces to metalwork, chromium adds color to emeralds and rubies. It exists as four stable (nonradioactive) isotopes with atomic masses of 50, 52, 53, and 54.

By making very accurate measurements of chromium isotopes in the meteorites compared to Earth rocks and comparing them to theoretical predictions, the researchers were able to show for the first time that lighter isotopes of chromium preferentially go into the core.

From this the team inferred that some 65 percent of the missing chromium is most likely in the Earth’s core. The work is detailed in the journal Science.

Researchers suspect the separation must have happened early in the planet-building process, probably in the multiple smaller bodies that assembled into the Earth or when the Earth was still molten but smaller than today.

Scientists from Washington University in St Louis and UCLA collaborated on the work, which was funded by NASA and the National Science Foundation.